Gas separation by pressure swing adsorption is achieved by coordinated pressure cycling and flow reversals over an adsorbent bed which preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorbent bed, and is reduced during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component in concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbent beds in parallel, with directional valving at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks, thus establishing the changes of working pressure and flow direction. Typically, the desired product is the less readily adsorbed fraction, which has been purified by substantial removal of the more readily adsorbed component. If the desired product component is present as a large fraction (greater than about 10% or 20%) of the feed gas mixture, and the adsorbent is sufficiently selective between the components, the desired product component can be purified to a high degree, although only incompletely recovered since some of the product is used to purge the adsorbent bed and is exhausted. If the component to be separated is present only as a small fraction (less than 1%) of the feed gas mixture, the conventional process cannot achieve satisfactory concentration of this component.
The conventional pressure swing adsorption process also makes inefficient use of applied energy, because of irreversible expansion over the valves while switching the adsorbent beds between higher and lower pressures.
The prior art includes pressure swing adsorption devices with improved capability for approaching complete separation of binary mixtures. Keller (U.S. Pat. No. 4,354,859) has disclosed a single bed pressure swing adsorption device, with mechanical volume displacement means cycling at the same frequency at both ends of the adsorbent bed, and with a specified range of phase angles between the two volume displacement means which are required to have unequal displacements, such that the displacement ratio of the smaller to the larger volume displacement means is in the specified range of about 0.15 to 0.65. The volume displacement means may be pistons or diaphragms. The feed mixture is introduced to an intermediate point of the adsorbent bed, and the product components are separated to either end. Keller showed experimentally that his device could achieve approximately complete separation of gas mixtures such as dry air and a 50/50 mixture of hydrogen and methane.
Other devices using cyclically operated volume displacement means at both ends of an adsorbent bed are disclosed in my U.S. Pat. No. 4,702,903 in which a temperature gradient is imposed on the adsorbent bed, my U.S. Pat. No. 4,801,308 in which the adsorbent bed is itself cyclically expanded and contracted, and my U.S. Pat. No. 4,816,121 which is concerned with separation of chemically reactive gases.
Relative to the above cited prior art, the present invention provides an improved process and apparatus for separating the components of binary gas mixtures with simultaneous substantial purification and high recovery of both components, particularly in applications where one component is initially present at low or trace concentrations.
While prior art adsorptive gas separation systems using a single working volume are capable of purifying a carrier gas and removing a trace gas phase component to a high degree, they have not been capable of simultaneously concentrating the trace component to a high degree, as is achieved in the present invention.